They may consist of millions of base pairs

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2 Restriction enzymes 2 DNA molecules are very long They may consist of millions of base pairs In order to study the structure of DNA, the molecules are broken up into smaller fragments by enzymes called restriction enzymes Restriction enzymes do not break up the DNA molecule randomly but cut it at particular sites

3 Restriction fragments 3 For example, a restriction enzyme called EcoR1* recognises the base sequence CAATTC and cuts it between the two As recognised --C-C-G-C-A-G-C-T-G-T-C-A-A-T-T-C-T-C-T-C-C-G-G-A-T-C-C-A --C-C-G-C-A-G-C-T-G-T-C-A cut A-T-T-C- T-C-T-C-C-G-G-A-T-C-C-A- Other restriction enzymes cut the DNA in different places and so produce fragments which are easier to analyse --C-C-G-C-A-G-C-T-G-T-C-A A-T-T-C-T-C-T-C-C-G-G-A-T-C-C-C-A- --C-C-G-C-A-G C-T-G-T-C-A A-T-T-C-T-C-C-G G-A-T-C-C-C-A-

4 4 The fragments cut by the restriction enzymes are called restriction fragments The fragments can be separated using gel electrophoresis (See slides 7 11)

5 Genetic fingerprinting 5 90% or more of DNA does not carry nucleotide triplets that code for proteins The non-coding DNA is often called junk DNA but this only means that its functions have not yet been discovered Some of the non-coding regions consist of repeated sequences of nucleotides For example -C-A-T-G-C-A-T-G-C-A-T-G-C-A-T-G- * The number of repeats in any one section of DNA varies from one individual to the next Genetic fingerprinting

6 Restriction enzymes 6 Particular repeat sequences can be cut out by restriction enzymes For example restriction enzyme cuts here and.....here -CATCCACGACATGCATGCATGCATGCCACATCCA- or here..... and......here -CCACGACATGCATGCATGCATGCATGCATGCCACAT-

7 Gel electrophoresis 7 The different sized fragments are separated by a process called gel electrophoresis The separation takes place in a sheet of a firm but jelly-like substance (a gel ) Samples of the DNA extracts are placed in shallow cavities ( wells ) cut into one end of the gel A voltage is applied to opposite ends of the gel DNA has a negative charge and moves slowly towards the positive end The shorter fragments travel through the gel faster than the longer fragments Gel electrophoresis

8 Gel electrophoresis 9 thin slab of gel Voltage supply DNA samples placed in wells cut in gel negative electrode positive electrode + DNA fragments Move from negative To positive

9 A sample with the shorter DNA fragments travels through the gel faster than a sample with the larger fragments 10

10 Next slide 11

11 Appearance of separated fragments on gel 12 These bands will contain the shorter DNA fragments These bands will contain the longer DNA fragments Prof. E. Wood Prof. E.J.Wood starting positions Appearance of bands

12 Genetic fingerprinting 13 DNA analysis can be used for catching criminals, establishing parentage, finding how closely organisms are related and many other applications. The pattern of bands in a gel electrophoresis is known as a genetic fingerprint or a genetic profile If a genetic fingerprint found in a sample of blood or other tissue at the scene of a crime matches the genetic fingerprint of a suspect, this can be used as evidence A DNA sample can be obtained from the suspect using blood, cheek epithelial cells taken from the mouth lining or even the cells clinging to the root of a hair Genetic fingerprinting

13 Chances of a match Suppose that 14.there is a chance of 1 in 10 that this fragment occurs in many individuals and.there is a chance of 1 in 20 that this fragment occurs in many individuals and.there is a chance of 1 in 10 that this fragment occurs in many individuals and.there is a chance of 1 in 30 that this fragment occurs in many individuals, but

14 Probability of a match 15 the probability of all 4 bands matching in any person other than the suspect is 1 in 10 x 1 in 20 x 1 in 10 x 1 in 30 = 1 in 10 x 20 x 10 x 30 That is 1 in 60,000 When a larger number of bands is involved, the probability that the suspect is not guilty becomes one in many thousands*

15 V S S 1 S 2 S 3 V Victim DNA profiles 16 S Sample from crime scene S 1 Suspect 1 S 2 Suspect 2 S 3 Suspect 3 More than 20 fragments from Suspect 1 match those taken from the crime scene